1. Field of the Invention
The present invention relates to a play-back device for optical discs of the so-called "carousel" type, in which a plurality of discs are mounted on the turntable of a tray. More particularly, the present invention relates to such a playback device in which any selected one of the plurality of discs may be randomly designated for playing, with a short access time.
2. Description of the Prior Art
FIGS. 5-7 show a carousel-type optical disc play-back device according to the prior art. In particular, FIG. 5 shows in perspective the inner workings of such a play-back device, in which the player is generally designated 1, and includes a front control panel 2. A tray 3 is provided which is caused to move in and out of the front panel 2 like a drawer, in response to actuation of a corresponding control button on the front panel 2 of the player 1. The tray 3 holds a plurality of discs 4, which in this example is five discs 4.
The tray 3 comprises a disc transport turntable 5. FIG. 6, which is an axial section through the disc transport and reading mechanisms of the player of FIG. 5, shows that the disc transport turntable 5 is mounted for rotation about a boss 6 which extends from the center of tray 3. The disc transport turntable 5 has a circular series of disc holding sections 7, corresponding in number to the number of discs 4 which may be held by the player, and it will be noted that each holding section 7 is capable of holding a disc of either of two different sizes.
On its underside, the disc transport turntable 5 is provided with an annular rib 8 which forms a rack by which to transmit a driving force for rotating the turntable 5, as well as two concentric annular ribs 9a and 9b disposed radially outwardly of the rib 8, and which are used to determine the angular position of the turntable 5.
As shown in FIG. 7, the position-determining rib 9a has notches 11 corresponding to each disc-holding section 7, to ensure that any selected disc 4 is correctly transferred to the disc rotation turntable 10, from its respective disc-holding section 7. FIGS. 6 and 7 show that a photocoupler 12 is so disposed as to straddle the annular rib 9a, for detecting the occurrence of a notch 11 and emitting a corresponding control signal, thereby to control the driving motor 13 which rotates the disc transport turntable 5 via gear 15 and the rack formed on annular rib 8.
In addition, a notch is formed on the outer annular rib 9b, to enable determination of a standard position of the disc transport turntable 5, using the output from a further photocoupler 14. Since the disc transport turntable 5 is rotated in a specified direction via annular rib 8, and the motor 13 and gear 15 installed on tray 3, an accurate position determination is produced, after rotating the turntable 5 by a predetermined angle, by having the next notch emit a control signal from photocoupler 12. In this manner, turntable 5 is gradually rotated to enable an exchange of discs 4 at the disc rotating turntable 10.
Each of the disc-holding sections 7 of the disc transport turntable 5 is provided with a radially-outwardly opening slot 16. When the tray 3 is stored inside the main body of the player 1, the opening 17 formed in the rear side of tray 3 is aligned with the slot 16 of one of the disc-holding sections 7, such that the disc rotation turntable 10 and optical pickup 18 are also axially aligned with the opening 17 and its aligned slot 16.
When the tray 3 is set at a predetermined position, the motor 19 shown in FIG. 6 is activated, and the output of motor 19 is transmitted through a gear set 20 to cause rotation of a cylindrical lift cam 21. Lift cam 21 is provided with a peripheral spiral groove 22 about its cylindrical outer wall, as well as an annular gear on its top for meshing with the aforementioned gear set 20.
Sub-chassis 23 shown in FIG. 6 carries the optical pickup 18 and disc rotation turntable 10, and is mounted for up and down pivotal movement about pin 25. Upon rotation of the lift cam 21, projection 24 secured to sub-chassis 23 is guided into the spiral groove 22, such that the sub-chassis 23 is guided in its up and down pivotal movement about pin 25. As the sub-chassis 23 is pivoted upwardly, the disc rotation turntable and optical pickup 18 rise and enter into opening 17 of tray 3 and slot 16 of the aligned disc holding section 7. With a further rise of sub-chassis 23, the disc rotation turntable 10 contacts the central hole of disc 4, causing disc 4 to separate from its disc-holding section 7. Subsequently, a disc clamp 27 extending downwardly from chassis 26 clamps disc 4 in such a manner that disc 4 faces turntable 10. At this point, disc turntable 10 commences rotation of disc 4, and data is retrieved from disc 4 by means of the optical pickup 18, in a known manner.
In the prior art disc player described above, after a disc has been placed in the disc transport turntable of the tray, verification of the standard position of the disc transport turntable must be performed before any disc can be played, by rotating the disc transport turntable and using the notches in the annular rib, together with the photocoupler, to establish the standard position. Next, the disc transport turntable must be further rotated such that the disc-holding section holding the selected disc is positioned above the optical reading device, by counting the number of notches in the position-determining annular rib. Only then may the selected disc be clamped on the disc rotation turntable and played.
Thus, the disc transport turntable must be rotated until the standard position is verified, before the selected disc can be transported to the playback components. Therefore, when, for example, the fifth disc in a carousel-type player capable of holding five discs is to be played first, verification of the standard position must first be accomplished by rotating the disc transport turntable to confirm the standard position (ordinarily the position of the holding section for the first disc). Next, the turntable is again rotated to position the selected disc above the disc rotation turntable and optical pickup. Since, in this example, the turntable rotates in a specified direction, a very long access time is required. Moreover, in order to solve this problem and to shorten the access time required in a conventional technique, there is the disadvantage that two sets of detection systems are necessary, one to detect the standard position of the disc transport turntable, and the other to detect the position of the selected disc.